Chance coincidence analysis for capture-gated neutron spectrometry with a composite scintillator

Neutron spectrometry is important for radiation metrology, radiation protection, accelerator physics etc. The spectral response of a capture-gated detector to mono-energetic neutrons contains a significant peak, making it easy to be analyzed. But this system output not only capture-gated events (CGE) but also chance coincidence events (CCE). It is critical to identify and eliminate CCE. However, chance coincidence has not been analyzed in detail before. This paper aims to find a way to eliminate CCE and get the spectrum of CGE. Thus, categories of CCE were analyzed and their probability equations were established. Then recorded pulses from a composite scintillator measured in mixed radiation fields were used to verify CCE and explore the influence of timing gate. Results show that n/gamma discrimination is effective to distinguish CCE caused by gamma-rays and calculate the amount of CCE caused by fast neutrons. Timing gate should be determined according to the different distributions of CGE and CCE. Spectrum of CGE can be acquired by subtracting that of fast neutron CCE from the fast neutron spectrum of coincidence events.